This invention relates to a radiation curable protective coating composition. More specifically, it relates to a coating composition which contains at least one multifunctional acrylate or methacrylate monomer and colloidal silica dispersed in water or in an organic solvent/water mixture. When applied to a substrate and radiation cured, the composition forms a transparent abrasion resistant, weather resistant, ultraviolet light resistant, coating firmly held thereon. In addition, substrates coated with this composition may be tinted and/or dyed.
Presently available radiation curable silicone coating compositions use silanes to stabilize and suspend silica. For example, U.S. Pat. No. 4,486,504, issued Dec. 4, 1984, discloses an ultraviolet radiation curable silicone coating composition containing a non-silyl acrylate, an acryloxy or glycidoxy functional silane, colloidal silica and a photoinitiator. U.S. Pat. No. 4,822,828 discloses an aqueous radiation curable coating composition containing a vinyl-functional silane which is the partial condensate of silica and a silane, and a multifunctional acrylate. In such compositions, the silanes undergo hydrolysis with the water present in aqueous dispersions of colloidal silica. Thus, the methoxy groups on the silane are replaced by hydroxy groups which hydrogen bond to, or form covalent bonds with, the hydroxy groups present on the surface of the silica particles. Thus, the silica particles remain suspended and do not precipitate in the presence of the hydrophobic acrylate monomers.
Silanes have been eliminated in certain radiation curable coating compositions. For example, Japanese Kokai Patent No. Sho 63[1988]-286409, issued Nov. 24, 1988, discloses a coating composition containing an .alpha., .beta.-unsaturated carboxylic acid compound and colloidal silica. In such a composition the acid group on the .alpha., .beta.-unsaturated carboxylic acid compound hydrogen bonds to, or forms covalent bonds with, the hydroxy groups present on the surface of the silica particles. Thus the silica particle remain suspended. Aqueous dispersions of colloidal silica, however, may not be used in Japanese Patent No. Sho 63[1988]-286409. Commonly assigned copending U.S. application Ser. No. 07/423,159, filed Oct. 18, 1989, also discloses a silane free coating composition wherein aqueous dispersions of colloidal silica may be used. This composition contains a hydroxy acrylate, colloidal silica and a multifunctional acrylate monomer. In this composition the hydroxy group on the hydroxy acrylate hydrogen bonds to, or forms covalent bonds with, the hydroxy groups present on the surface of the silica particles. Thus the silica particles remain suspended and do not precipitate or gel.
The present inventors have discovered a silane-free coating composition in which aqueous dispersions of colloidal silica may be used and which does not require hydroxy functional groups to stabilize the silica particles.